The HODL chart illustrates the last time a UTXO has moved. It does not illustrate how long one has held bitcoin.

If you had ₿10 in 6 UTXOs for 10 years, and you decide to adjust your distribution of the bitcoin across those UTXOs, the day you make that adjustment the 10 year counter on those touched will be reset to 0.

If you want to be deep on the HODL chart, you must have untouched UTXOs for that period of time. This requires UTXO management for some, all or 1 UTXO has the chance to age.